Update Notebook dts (#3)

Co-authored-by: TylerLeonhardt <[email protected]>

Author: github-actions[bot]

vscode.proposed.d.ts

@@ -3,6 +3,8 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 
+import { MarkdownString } from 'vscode';
+
 /**
  * This is the place for API experiments and proposals.
  * These API are NOT stable and subject to change. They are only available in the Insiders